Gallium Arsenide (GaAs) offers many advantages over silicon (Si) in terms of increased speed and radiation hardness, and its ability to process and transmit optical signals. Silicon, on the other hand, is a well established material for integrated circuits and has superior mechanical and thermal characteristics. By growing epitaxial layers of GaAs on a Si substrate, it is possible to combine the advantages of both materials. Presently, GaAs/Si heterostructures grown by organometallic chemical vapor deposition (MOCVD) have defect densities of about 10.sup.6 cm.sup.-2, compared with 10.sup.2 -10.sup.3 cm.sup.-2 for silicon and 10.sup.3 -10.sup.4 cm.sup.-2 for bulk GaAs. These heterostructures are adequate for some electronic device applications, but for high performance devices, such as light emitting diodes (LEDs) and lasers, defect densities at least of the order of 10.sup.4 -10.sup.5 cm.sup.-2 or lower are required.
Threading dislocations and other defects in GaAs/Si heterostructures are primarily caused by lattice mismatch between the GaAs and Si layers (a.sub.Si =5.43 A, a.sub.GaAs =5.65 A), thermal mismatch (coefficient of thermal expansion .alpha..sub.Si =2.6.times.10.sup.-6 k.sup.-1, .beta..sub.GaAs =5.8.times.10.sup.-6 K.sup.-1, and growth related defects. Several methods have been employed to reduce the threading dislocation density in lattice mismatched epilayers such as the introduction of strained layer superlattices in the epilayer. A strained layer superlattice comprises an ordered arrangement of atoms of a first material disposed on a second material and forming a superimposed lattice on the lattice of the second material. The strain present in the superlattice is due to lattice mismatch between the two materials. In a structure employing a superlattice, the strain causes threading dislocations extending into the superlattice from a lower portion of the epilayer to bend. This bending prevents propagation through the superlattice to an upper portion of the epilayer. In one experiment, a GaAsP/GaAs strained layer superlattice was employed for dislocation density reduction in the growth of GaAs on a GaAs substrate. However, a need exists for new processes that will further reduce the density of dislocations in gallium arsenide on silicon structures.